For the past three decades intense efforts by numerous investigators aimed at the design of immunogens that would stimulate the production of broadly neutralizing antibodies (bnAbs). Although improvements were made in the design/engineering, expression and purification of Env-based, in general the neutralizing antibody responses elicited by such immunogens are narrow in breadth and potency. One of the known targets of bnAbs is the CD4-BS of the HIV Env. Regarding the inability of recombinant Envs to elicit anti- CD4-BS, our studies suggest that one reason for this is that Env immunogens do not engage the germline forms of known anti-CD4-BS bnAbs and of the corresponding germline B cell receptors (BCRs). We believe that a direct consequence of this lack of Env-BCR reactivity is that the process of production of such antibodies is not initiated by such Env immunogens. Our proposed studies are based on our recent design of a trimeric gp140 clade C Env protein that engages the germline VRCOI and NIH45-46 antibodies (two of the most potent and broadly neutralizing anti-CD4-BS antibodies known) and activate B cells that express the corresponding BCR forms of these antibodies. We believe, therefore, that we identified a way to overcome the earliest roadblock in the elicitation of such bNAbs. As such, our findings are very novel and highly significant to efforts to develop by immunization bNAbs against HIV. In this Project One, we propose to further optimize our immunogens to improve their ability to engage and activate the germline BCRs of known anti-CD4-BS bNAbs. This Project One has three Specific Aims: 1) Optimize the design of our current immunogens to expand their germline BCR-activation capabilities; 2) Engineer and characterize a modified variant of our immunogen that lacks specific variable domains, and 3) Design/optimize immunogens for macaque BCR activation. The proposed research in Project One is central to the overall activities and aims of our IPCAVD Program.